Due to governmental legislation as mandated in the Renewable Fuels Standards (RFS), there is an increasing need for biofuels fungible at high concentrations with current transportation fuels. The bio-oils obtained by pyrolysis of biomass or waste have received attention recently as an alternative source of fuel.
Pyrolysis is the chemical decomposition of organic materials by heating in the absence of oxygen or other reagents. Pyrolysis can be used to convert biomass (such as lignocellulosic biomass) into pyrolysis oil or so-called bio-oil.
The major advantage of these fuels is that these are CO2 neutral and contain a very low fraction of bonded sulfur and nitrogen. Thus, they contribute very little to the emission of greenhouse gases or other regulated air pollutants.
There has been a considerable effort in the past to develop pyrolysis processes for the conversion of biomass and waste to liquids for the express purpose of producing renewable liquid fuels suitable for use in boilers, gas turbines and diesel engines.
However, pyrolysis oil (or bio-oil) produced from pyrolysis of biomass is a chemically-complex mixture of compounds comprising generally a mixture of water, light volatiles, and non-volatiles. As a fuel, pyrolysis oil has a number of negative properties such as high acidity (lead to corrosion problem), substantial water content (usually in the range of 15% to 30%), variable viscosity, low heating values (about half that of the diesel fuel), low cetane number, etc. These negative properties are related to the oxygenated compounds contained in bio-oils that result in an oxygen content of approximately 45 wt %. Therefore, it is necessary to upgrade the raw bio-oils before they can be used as a viable and regular fuel.
The upgrading of pyrolysis oils, a necessary process before use as a regular fuel, essentially involves the removal of oxygen. Particular attention has been focused on hydrotreating using conventional petroleum refining catalysts, for example, cobalt-molybdenum or nickel-molybdenum on alumina to produce essentially oxygen-free naphthas. Since pyrolysis liquids typically contain oxygen at 30 to 50 wt %, complete removal of oxygen requires a substantial consumption of hydrogen, estimated to be as much as 600 to 1000 L/kg of pyrolysis liquid [W. Baldauf, et al., 7th European Conference on Biomass for Energy and Environment, Agriculture and Industry]. This method represents a major and prohibitive cost.
Currently, there is no commercial technology that allows obtaining fungible biofuels from pyrolysis oil in large volumes to compete with current established biofuels.
Therefore, developing a new method or process for upgrading the bio-oils obtained by pyrolysis of biomass or waste to thermally stable and fungible renewable fuels would be a significant contribution to the art.